There are known various conventional quantitative voltage contrast measurement systems for scanning electron microscopes and electron beam testing, such as that described in "An Analysis of the Local Field Effect Probe Voltage Measurements", Nakamura et al., 1993, Scanning Electron Microscopy, Pages 1187-1195; "VLSI Testing Using the Electron Probe", H. P. Feuerbaum, Scanning Electron Microscopy, 1979, pages 285-296; and "Local Field Effects on Voltage Measurement Using a Retarding Field Analyser in the Scanning Electron Microscopy", Fujioka et al., Scanning Electron Microscopy, 1981, pages 323-332, which use feedback techniques. There are other known systems such as that described in "The effect of passivation on the observation of voltage contrast in the scanning electron microscope", D. M. Taylor, The Institute of Physics, Vol. 11, 1978, pages 2443-2454; and "A voltage contrast detector for the SEM", Hardy et al., Journal of Physics E: Scientific Instruments, Vol. 8, Mar. 10, 1975, pages 789-793, which use the peak detection method. These known systems suffer from poor accuracy due to various error components like the potential barrier effect, off-normal incidence injection of secondary electrons into the analyzing field, lens effect, and analyzer geometry effects. These systems require a high extraction field (500 V/nm to 1000 V/nm) to minimize the error components due to local field effects (both Types I and II) acting on secondary electron trajectories. Unfortunately, the use of these high extraction fields is not compatible with certain sensitive specimens or with passivated specimens.
Furthermore, conventional systems cannot perform accurate voltage measurements on underlying structures in multi-level component systems due to errors from charging effects (e.g., in a multi-level component system where underlying metal structure may be exposed by focused ion beam milling with walls of insulator material surrounding the point of measurement).